A&A 626, A85 (2019) Astronomy https://doi.org/10.1051/0004-6361/201834350 & c ESO 2019 Astrophysics Starburst and post-starburst high-redshift protogalaxies The feedback impact of high energy cosmic rays Ellis R. Owen1,2, Kinwah Wu1,3, Xiangyu Jin4,5, Pooja Surajbali6, and Noriko Kataoka1,7 1 Mullard Space Science Laboratory, University College London, Dorking, Surrey RH5 6NT, UK e-mail: [email protected] 2 Institute of Astronomy, Department of Physics, National Tsing Hua University, Hsinchu, Taiwan (ROC) 3 School of Physics, University of Sydney, Camperdown, NSW 2006, Australia 4 School of Physics, Nanjing University, Nanjing 210023, PR China 5 Department of Physics and McGill Space Institute, McGill University, 3600 University St., Montreal, QC H3A 2T8, Canada 6 Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, Heidelberg 69117, Germany 7 Department of Physics, Faculty of Science, Kyoto Sangyo University, Kyoto 603-8555, Japan Received 1 October 2018 / Accepted 27 April 2019 ABSTRACT Quenching of star-formation has been identified in many starburst and post-starburst galaxies, indicating burst-like star-formation histories (SFH) in the primordial Universe. Galaxies undergoing violent episodes of star-formation are expected to be rich in high energy cosmic rays (CRs). We have investigated the role of these CRs in such environments, particularly how they could contribute to this burst-like SFH via quenching and feedback. These high energy particles interact with the baryon and radiation fields of their host via hadronic processes to produce secondary leptons. The secondary particles then also interact with ambient radiation fields to generate X-rays through inverse-Compton scattering. In addition, they can thermalise directly with the semi-ionised medium via Coulomb processes. Heating at a rate of ∼10−25 erg cm−3 s−1 can be attained by Coulomb processes in a star-forming galaxy with one core-collapse SN event per decade, and this is sufficient to cause quenching of star-formation. At high-redshift, a substantial amount of CR secondary electron energy can be diverted into inverse-Compton X-ray emission. This yields an X-ray luminosity of above 1041 erg s−1 by redshift z = 7 which drives a further heating effect, operating over larger scales. This would be able to halt inflowing cold gas filaments, strangulating subsequent star-formation. We selected a sample of 16 starburst and post-starburst galaxies at 7 . z . 9 and determine the star-formation rates they could have sustained. We applied a model with CR injection, propagation and heating to calculate energy deposition rates in these 16 sources. Our calculations show that CR feedback cannot be neglected as it has the strength to suppress star-formation in these systems. We also show that their currently observed quiescence is consistent with the suffocation of cold inflows, probably by a combination of X-ray and CR heating. Key words. astroparticle physics – galaxies: ISM – cosmic rays – X-rays: galaxies – galaxies: evolution 1. Introduction emission lines in the central regions of post-starburst outflows, for example Yang et al. 2006; Yan et al. 2006; Tremonti et al. Post-starburst galaxies are systems in which star-formation 2007). appears to have been rapidly quenched after a violent starburst The presence of the large molecular reservoir, dust and a episode (Dressler & Gunn 1983; Couch & Sharples 1987). Such substantial interstellar medium (ISM) in many of these sys- systems are seen both at low redshift (Dressler & Gunn 1983; tems disfavours the action of outflows (Roseboom et al. 2009; Couch & Sharples 1987; French et al. 2015, 2018; Rowlands Rowlands et al. 2015; Alatalo et al. 2016; French et al. 2018; et al. 2015; Alatalo et al. 2016) and high redshift (Watson et al. Smercina et al. 2018). However, a deficiency of dense molecu- 2015; Hashimoto et al. 2018; Owen et al. 2019) and the abun- lar gas pockets recently observed in low-redshift post-starburst dant reservoirs of interstellar molecular gas often found in these systems (French et al. 2018) implies the operation of processes objects would ordinarily enable vigorous star-formation to con- able to provide additional pressure support to the molecular gas tinue – however, it instead appears to be quenched (French et al. reservoir against fragmentation and collapse. There is debate 2015; Rowlands et al. 2015; Alatalo et al. 2016). Researchers have over the exact process which may be at work here but, if it therefore started to explore alternative mechanisms that would is feedback from the starburst episode itself, a promising can- explain the observed quiescence. Proposed mechanisms include: didate would be the action of high energy (HE) cosmic rays starvation (Gunn & Gott 1972; Cowie & Songaila 1977; Nulsen (CRs): these are able to directly provide additional pressure sup- 1982; Moore et al. 1999; Boselli & Gavazzi 2006), strangulation port (e.g. Salem & Bryan 2014; Zweibel 2016), as well as drive (Larson et al. 1980; Boselli & Gavazzi 2006; Peng et al. interstellar heating effects (e.g. Begelman 1995; Zweibel 2013, 2015), feedback from the starburst and associated stellar end- 2016; Owen et al. 2018) so as to also increase thermal pres- products (Goto 2006; Kaviraj et al. 2007; French et al. 2015), sure support. Indeed, such CR heating would be particularly and the removal of dense molecular gas in outflows (Narayanan focussed in denser regions of the host galaxy’s ISM. This is et al. 2008; potential evidence of which would be low-ionisation where increased thermal pressure support would be particularly Article published by EDP Sciences A85, page 1 of 28 A&A 626, A85 (2019) required to hamper gravitational collapse and halt the formation than the measured star-formation rate. Hence, the observed star- of the dense pockets required for star-formation. formation must be relatively suppressed compared to the earlier Not only do CRs provide the required feedback channels, episode during which the bulk of the stellar population would they are also known to be abundant in starburst galaxies – evi- have formed. Moreover, the star-formation history (SFH) must dence of which includes the bright γ-ray glow of, for example, have been burst-like but subsequently quenched – not unlike the Arp 220 (e.g. Griffin et al. 2016; Peng et al. 2016; Yoast-Hull SFHs inferred from closer post-starburst systems (French et al. et al. 2017), NGC 253 and M82 (Acero et al. 2009; VERITAS 2015, 2018; Rowlands et al. 2015). Collaboration 2009; Abdo et al. 2010a; Rephaeli & Persic 2014) A number of similar high-redshift galaxies were found to and M31 (Abdo et al. 2010b) among others (e.g. Abdo et al. exhibit burst-like SFHs, and we have identified 11 other exam- 2010c,d; Hayashida et al. 2013; Tang et al. 2014; Rojas-Bravo ples (see Table1). These systems tend to show a relatively & Araya 2016) which results from CR interactions. The source high stellar mass combined with a relatively low inferred star- of these CRs is thought to be stellar end products, for exam- formation rate, being high-redshift galaxies in a post-starburst, ple supernova (SN) remnants (Hillas 1984; Ackermann et al. quenched stage of their evolution. If enforcing such an assump- 2013; Kotera & Olinto 2011; Morlino 2017), which can acceler- tion on their SFH, sufficient information is available in these ate CRs to relativistic energies by, for example, Fermi processes 12 cases to estimate the timescale over which their rapid star- (Fermi 1949). The high star-formation rates of starburst pro- formation episode progressed, if also adopting an upper estimate togalaxies leads to frequent core-collapse SN explosions from for the redshift of formation of the first stars in these galax- their rapidly ageing population of massive, low-metallicity Type ies, zf. Recent studies (e.g. Hashimoto et al. 2018) estimate this +2:7 O and B stars. As such, starburst environments are effective CR redshift to be around zf = 15−2:0 (see also Thomas et al. 2017 factories (Karlsson 2008; Lacki et al. 2011; Lacki & Thompson which suggests a similar but slightly lower redshift of formation 2012; Wang & Fields 2014; Farber et al. 2018), and it would of zf = 14:8), which would correspond to a time when the age therefore not be surprising for CRs to have an important role in of the Universe was around 250–300 Myr (Wright 2006). It is feedback and quenching, both in high-redshift protogalaxies as likely that this would be an extreme case for the formation of the well as their more local counterparts. earliest galaxies, so could reasonably be regarded as a lower age This study particularly considers the role of CRs in high- limit, with many galaxies forming their first stars at later times – redshift protogalactic environments, when these systems were for example, evidence suggests a later zf ≈ 9 for A1689-zD1 is forming their first populations of stars. We organise this paper more appropriate (Mainali et al. 2018). as follows: in Sect.2 we consider the indications of bursty star- It is also informative to consider certain ongoing starbursts formation histories and the presence of quenching in starburst at high-redshift where observations allow for the star-formation and post-starburst systems observed at high redshift. We then rate to be found, and the characteristic stellar population age to introduce a parametric model of a characteristic protogalaxy be estimated to give indications as to the duration of the active environment in Sect.3, in which we consider the injection and starburst. We have been able to identify four examples where this interactions of CRs. In Sect.4, we model CR propagation in pro- is already possible from the literature (see Table2). This pro- togalaxies, accounting for their interactions, secondaries, cool- vides us with information about pre-quenched systems as well ing and the ambient magnetic field.